1. Field of the Invention
The invention relates to a metal foil for secondary battery and a secondary battery.
2. Description of the Related Art
In recent years, along with the popularization of portable devices such as a cell phone or a laptop computer and along with the development and practical application of an electric vehicle or a hybrid car, demand of a compact high-capacity battery has been increasing. Especially a lithium-ion battery has been used in various fields due to its lightweight and high energy density.
The lithium-ion battery is basically composed of a cathode, an anode, a separator for insulating the cathode from the anode, and an electrolyte which permits ion movement between the cathode and the anode.
In general, a metal foil formed of a band-shaped aluminum foil in which an active material such as lithium cobalt oxide is applied to front and back surfaces thereof is used as the cathode. Meanwhile, a metal foil formed of a band-shaped copper foil in which an active material such as a carbon material is applied to front and back surfaces thereof is used as the anode.
When the active material is applied to the front and back surfaces of the copper or aluminum metal foil, there is a disadvantage that the metal foil is less likely to be integrated with the active material and the active material is likely to fall off. Therefore, a method is known in which the active material is prevented from falling off by forming a through-hole on the metal foil and integrating the active materials applied to the front and back surfaces of the metal foil through the through-hole (see JP-A 2002-198055).
Plural conical through holes are formed on front and back surfaces of the metal foil by a rolling process in which the metal foil is passed between a pair of rollers on which a conical convex portion is formed, thereby forming the metal foil in a three-dimensional shape.
Meanwhile, a method of forming through-holes by a lath process is known in which cuts are formed in a thin plate and the thin plate is extended in a direction orthogonal to the cutting direction (e.g., see JP-A 2002-216775).
However, since cutting chips are generated in the method of forming through-holes by the rolling process, removal of the cutting chips is required. Meanwhile, in the method of forming through-holes by the lath process, since strength against tension generated at the time of applying the active material is insufficient when the expansion rate is increased, it is not possible to increase the aperture ratio.